1. Homework #10 is due tonight, 9:00 pm.
Review session tonight, 7:15 pm
in classroom
Final Exam, Wednesday, May 2,
12:30-2:30 p.m., Swain West 119

2. Estimated age depends on both dark matter and dark energy

3. Will the universe continue expanding forever?
Current measurements indicate that there is not enough dark matter to prevent the universe from expanding forever
Is the expansion of the universe accelerating?
An accelerating universe is the best explanation for the distances we measure when using white dwarf supernovae as standard candles

4. The Beginning of Time What were conditions like in the early universe?
What is the history of the universe according to the Big Bang theory?

5. The early universe must have been extremely hot and dense

6. Photons converted into particle-antiparticle pairs and vice-versa
E = mc2
Early universe was full of particles and radiation because of its high temperature

7. Forces were unified at these high temperatures
Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity

8. What is the history of the universe according to the Big Bang theory?

9. Planck Era
Before Planck time (~10-43 sec)
No theory of quantum gravity
GUT Era
From Planck time (~10-43 sec) to end of GUT force (~10-38 sec)
Electroweak Era
Lasts from end of GUT force (~10-38 sec) to end of electroweak force (~10-10 sec)

10. Particle Era
10-10 – sec
Amounts of matter and antimatter nearly equal
(Roughly 1 extra proton for every 109 proton-antiproton pairs!)

11. Era of
Nucleosynthesis
0.001 sec – 5 min
Begins when matter annihilates remaining antimatter at
~ sec
Nuclei begin to fuse, Helium formed

12. Era of Nuclei
5 min – 380,000 yr
Universe becomes too cool to blast helium apart but still too hot for electrons to combine with nuclei. Plasma and free electrons.
Radiation trapped in “fog” of free electrons

13. Era of Atoms
380,000 yr – 1 billion yr
Atoms form at age ~ 380,000 years
Cool enough for electrons to combine with atomic nuclei.
Background radiation released

14. Era of Galaxies
Galaxies form at age ~ 1 billion years

15. What were conditions like in the early universe?
The early universe was so hot and so dense that radiation was constantly producing particle-antiparticle pairs and vice versa
What is the history of the universe according to the Big Bang theory?
As the universe cooled, particle production stopped, leaving matter instead of antimatter
Fusion turned remaining neutrons into helium
Radiation traveled freely after formation of atoms

16. Primary Evidence for Big Bang
The Hubble Law
We have detected the leftover radiation from the Big Bang.
The Big Bang theory correctly predicts the abundance of helium and other light elements.

17. Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

18. The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965

19. Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves
The COBE satellite, designed by John Mather, successfully looked for the microwave background.
(This led to Mather’s Nobel Prize)

20. Background has perfect thermal radiation spectrum at temperature 2
Background has perfect thermal radiation spectrum at temperature 2.73 K
Dots: COBE observed spectrum
Line: Theoretical prediction

21. How do we observe the radiation left over from the Big Bang?
Radiation left over from the Big Bang is now in the form of microwaves—the cosmic microwave background—which we can observe with a radio telescope.
How do the abundances of elements support the Big Bang theory?
Observations of helium and other light elements agree with the predictions for fusion in the Big Bang theory

22. Mysteries Needing Explanation
Where does structure come from?
Why is the overall distribution of matter so uniform?
Why is the density of the universe so close to the critical density?

23. Mysteries Needing Explanation
Where does structure come from?
Why is the overall distribution of matter so uniform?
Why is the density of the universe so close to the critical density?
An early episode of rapid inflation can solve all three mysteries!

24. The inflationary model
Theory suggests that the universe experienced a brief period of inflation shortly after the Planck time.
Planck time: First s of the lifetime of the universe.
During inflation the universe expanded by a factor of 1050 in about s!

25. This doesn’t violate the idea that nothing can travel faster than the speed of light. The great increase in distance between objects was not due to motion but to the expansion of space.

26. Inflation can make all the structure observed in the microwave background by stretching tiny quantum ripples to enormous size
These ripples in density then become the seeds for all physical structures

27. How can microwave temperature be nearly identical on opposite sides of the sky?

28. Regions now on opposite sides of the sky were close together before inflation pushed them far apart

29. Density = Critical
Overall geometry of the universe is closely related to total density of matter & energy
Density > Critical
Density < Critical

30. Inflation of universe flattens overall geometry like the inflation of a balloon, causing overall density of matter plus energy to be very close to critical density

31. Inflation is testable!

32. Patterns of structure observed by WMAP show us the “seeds” of universe (small scale fluctuations in density)

33. Observed patterns of structure in universe agree (so far) with the “seeds” that inflation would produce

34. “Seeds” Inferred from Cosmic Microwave Background (CMB)
Overall geometry is flat
Total mass+energy has critical density
Ordinary matter ~ 4.4% of total
Total matter is ~ 27% of total
Dark matter is ~ 23% of total
Dark energy is ~ 73% of total
Age of 13.7 billion years

35. “Seeds” Inferred from Cosmic Microwave Background (CMB)
Overall geometry is flat
Total mass+energy has critical density
Ordinary matter ~ 4.4% of total
Total matter is ~ 27% of total
Dark matter is ~ 23% of total
Dark energy is ~ 73% of total
Age of 13.7 billion years
In excellent agreement with observations of present-day universe and models involving inflation and WIMPs!

36. How does inflation explain these features?
What aspects of the universe were originally unexplained with the Big Bang theory?
The origin of structure, the smoothness of the universe on large scales, the nearly critical density of the universe
How does inflation explain these features?
Structure comes from inflated quantum ripples
Observable universe became smooth before inflation, when it was very tiny
Inflation flattened the curvature of space, bringing expansion rate into balance with the overall density of mass-energy

37. How can we test the idea of inflation?
We can compare the structures we see in detailed observations of the microwave background with predictions for the “seeds” that should have been planted by inflation
So far, our observations of the universe agree well with models in which inflation planted the “seeds”

38. Why is the darkness of the night sky evidence for the Big Bang?

39. Olbers’s Paradox
If universe were
1) infinite
2) unchanging
3) everywhere
the same
Then, stars would cover the night sky

40. Olbers’s Paradox
If universe were
1) infinite
2) unchanging
3) everywhere
the same
Then, stars would cover the night sky

41. Night sky is dark because the universe changes with time
As we look out in space, we can look back to a time when there were no stars

42. Night sky is dark because the universe changes with time
As we look out in space, we can look back to a time when there were no stars

43. Why is the darkness of the night sky evidence for the Big Bang?
If the universe were eternal, unchanging, and everywhere the same, the entire night sky would be covered with stars
The night sky is dark because we can see back to a time when there were no stars